For many years, corrosion inhibiting organic coatings have been applied to metal coils or sheets prior to forming into finished articles. Designing with prepainted metal provides the metal finisher with many benefits, such as elimination of in-house painting operations, reduction in associated environmental liabilities, and improvement in the quality of the paint finish. One of the problems encountered in using prepainted metal is that if such articles are to be assembled, they must be joined together by adhesives or weldless fasteners, since organic coatings are insulative in nature and are either not weldable or weldable with difficulty and only by employing special techniques and equipment.
These techniques include spot welding with higher currents or longer weld times. However, such unorthodox methods are time-consuming and costly. Also, excessive temperatures are normally generated in the weld areas, which, in turn, causes vaporization and expulsion of the metal out from between the welding electrodes. This results in inferior welds as well as rapid deterioration of the electrode tips. Other techniques include decreasing the thickness of the protective film which sacrifices corrosion protection for weldability.
Recently, there has been a growing demand for "weldable" organic coatings. Organic coatings which are electroconductive and allow for electric resistance welding through their cured coating films without resorting to special equipment and techniques are said to be "weldable" or have weld-through capability. Various types of weldable anticorrosive liquid coatings have been proposed which typically contain conductive powdered metals or alloys to reduce the electrical resistivity of the coating film U.S. Pat. No. 5,001,173 (Anderson et al.) discloses some commercially popular weldable primers which contain high concentrations of conductive powdered ferroalloys, such as ferrophosphorous (a mixture containing di-iron phosphide and iron phosphide), and powdered zinc.
Zinc powder alone is not considered a good welding aid. Moreover, one of the problems encountered with ferrophos-rich weldable coatings in their appearance. Ferrophos is a very dark gray pigment, and when provided in the coatings in the high pigment to binder ratio necessary to impart desired weldability, it tends to produce very dark gray colored films, which are undesirable in certain applications. For instance, mar resistance is almost nil and even fingernail scratches are highly visible. In addition, the dark gray coating film tends to detract from the appearance of any topcoat finish applied thereover. Usually, it is necessary to topcoat at high film builds for adequate hiding or encryption of the primer, which, in turn, is very costly. Attempts have been made to lighten weldable primers to improve mar resistance and cryptability by adding standard light colored pigments, such as titanium dioxide, without much success. The standard pigments are inhibitively insulative, and the high pigment concentration needed to offset the darkness tends to impair weldability.
One solution to this problem has been to return to the use of standard non-weldable mar resistant coatings. Yet without welding aids in the formulations, the very thin films (i.e., no greater than about 0.1 mil thick) required for weldability is usually below the minimum thickness needed to provide adequate film opacity and corrosion resistance. Another approach taken has been to use a two-coat weldable primer as disclosed in U.S. Pat. No. 5,260,120 (Moyle et al.), wherein a ferrophos-rich primer is top coated with an extremely thin layer of a non-weldable, titanium dioxide-rich, protective coating. The thin protective film provided does not significantly interfere with the weldability of the conductive primer, yet provides a light colored surface film which has greatly improved mar resistance. The protective film also smooths out the abrasiveness of the underlying ferrophos primer. However, it is time-consuming and costly to employ such a two-step coating procedure.
Another problem encountered with weldable ferrophos-rich primers is their abrasiveness, which raises excessive stamping and forming die wear concerns during metal finishing operations. The abrasive, sandpaper, texture of the film finish is due to the hardness of the ferrophos. As mentioned above, the Moyle et al. patent provides a solution to this problem but again requires an undesirable two-step coating procedure.
A further problem with ferrophos-rich primers is that during welding they produce toxic fumes, such as phosphine gas, along with objectionable odors when subject to the required welding temperatures. While the toxicity does not reach the environmentally harmful and physiologically unsafe levels, workers have been known to complain about unpleasant odors produced during welding. It is difficult, or course, to reduce toxic effluents and eliminate unpleasant odors produced by ferrophos primers without sacrificing weldability.
Still another problem encountered with ferrophos-rich primers is that the film finish has a very high coefficient of friction. During metal finishing, the stamping and forming dies tend to scape off the coating film. Corrosion protection in these areas is thus lost. Also, the paint scrapings tend to build-up and eventually cause the finishing line to shut down. Internal lubricants, such as polytetrafluoroethylene, have been incorporated in conductive coatings to lower surface friction, allowing the finishing operations to proceed without destroying the coating, as disclosed in U.S. Pat. No. 5,624,978 (Soltwedel et al.).
Weldable primers also invariably shorten the life of the welding electrodes. Copper tipped electrodes on resistance spot welders are easily degraded by coating pick-up during welding. The number of spot welds that can be made on precoated metal before corrective action is required is dramatically reduced in comparison to that for bare metal. This results in reduced productivity arising from the need to change or dress the electrodes more frequently as well as inconsistent weld quality. Weldable coatings which extend the electrode life are continually being sought.
Other types of weldable liquid coatings have been disclosed which contain metallic welding aids other than ferrophos or zinc powders, but all of them suffer from drawbacks. For example, U.S. Pat. No. 5,047,451 (Barrett et al.) discloses a weldable liquid anticorrosive primer containing a welding agent of powdered nickel, a non-weldable corrosion inhibitor of powdered aluminum or stainless steel, a polyethylene suspension agent for preventing the finely divided metal from settling out, a silane-treated silicon dioxide thixotropic agent, a drawing agent of polytetrafluoroethylene, and a hygroscopic agent. Nickel powder, however, is dark gray and thus undesirable for improving mar resistance and topcoat crypt. Nickel powder is also an expensive material and uneconomical for use in weldable coatings.
Earlier U.S. Pat. No. 2,666,835 (Elleman) discloses a weldable liquid anticorrosive zinc chromate primer containing up to 30 vol. % of primer solids of a non-oxidized, magnetic, metal powder, such as non-oxidized forms of nickel powder, soft iron powder, stainless steel powder, steel powder, and nickel alloy powder. Nickel powder, however, is clearly preferred due to its inherent possession of magnetic remanence, which causes the metal particles to naturally link together and form conductive chains in the paint film. While coatings containing soft iron powder are mentioned, Elleman suggests the need for chemically reducing the thin oxide layer normally present on iron powder before incorporating it in the coating. This special procedure, for inclusion of only substantially non-oxidized soft iron powder, is time-consuming and costly.
Elleman also resorts to other special techniques for generating the weldable coating. For instance, Elleman suggests the need to expose the liquid coating to a magnetic field prior to drying on metal, in order to uniformly align the metal particles and thus impart the necessary conductivity to the film. This adds a time-consuming step to the welding process which, in turn, leads to reduced productivity and increased costs. These primers also require zinc chromate. While chromate pigments, including zinc chromate, strontium chromate, calcium chromate, and lead chromate, are excellent corrosion inhibitors, they are bright yellow insulative pigments and tend to produce darker coatings having reduced mar resistance and higher topcoat crypt.
What is needed is a weldable liquid corrosion inhibiting coating which forms a dry, electroconductive film on metal substrates which has improved mar resistance, improved topcoat crypt, reduced abrasiveness, reduced friction, reduced toxic and unpleasant odor emissions, extended electrode life, without sacrificing weldability and corrosion resistance, and that can weld together, in its cured state, two pieces of metal, such as steel, coated with the same, without the need for special equipment and techniques.